docs: def load(filename):
          """Load a RigidTransform from a file.
  
          The file format is:
          from_frame
          to_frame
          translation (space separated)
          rotation_row_0 (space separated)
          rotation_row_1 (space separated)
          rotation_row_2 (space separated)
  
          Parameters
          ----------
          filename : :obj:`str`
              The file to load the transform from.
  
          Returns
          -------
          :obj:`RigidTransform`
              The RigidTransform read from the file.
  
          Raises
          ------
          ValueError
              If filename's extension isn't .tf.
          """
          file_root, file_ext = os.path.splitext(filename)
          if file_ext.lower() != TF_EXTENSION:
              raise ValueError('Extension %s not supported for RigidTransform. Can only load extension %s' %(file_ext, TF_EXTENSION))
  
          f = open(filename, 'r')
          lines = list(f)
          from_frame = lines[0][:-1]
          to_frame = lines[1][:-1]
  
          t = np.zeros(3)
          t_tokens = lines[2][:-1].split()
          t[0] = float(t_tokens[0])
          t[1] = float(t_tokens[1])
          t[2] = float(t_tokens[2])
  
          R = np.zeros([3,3])
          r_tokens = lines[3][:-1].split()
          R[0, 0] = float(r_tokens[0])
          R[0, 1] = float(r_tokens[1])
          R[0, 2] = float(r_tokens[2])
  
          r_tokens = lines[4][:-1].split()
          R[1, 0] = float(r_tokens[0])
          R[1, 1] = float(r_tokens[1])
          R[1, 2] = float(r_tokens[2])
  
          r_tokens = lines[5][:-1].split()
          R[2, 0] = float(r_tokens[0])
          R[2, 1] = float(r_tokens[1])
          R[2, 2] = float(r_tokens[2])
          f.close()
          return RigidTransform(rotation=R, translation=t,
                                from_frame=from_frame,
                                to_frame=to_frame)